Duplex stainless steel has the two phases, an austenite phase and a ferrite phase, in the micro-structure of the steel and has been used as a high strength, high corrosion resistance material since the past for materials for petrochemical facilities, materials for pumps, materials for chemical tanks, etc. Further, duplex phase stainless steel is generally made of a composition with low Ni, so along with the recent skyrocketing price of metal raw materials, it is being closely looked at as a material with a lower and less fluctuating alloy cost compared with an austenitic stainless steel that is the mainstream of stainless steel.
As recent topics in duplex stainless steel, there are the development of lean types and their increased amount of usage. A “lean type” is a type of steel in which the content of expensive alloy elements is kept down compared with the conventional duplex stainless steels and the merit of low alloy cost is further enhanced. This is disclosed in Japanese Patent Publication (A) No. 61-56267, WO2002/27056, and WO96/18751. Among these, the duplex stainless steels disclosed in Japanese Patent Publication (A) No. 61-56267 and WO2002/27056 have been standardized in ASTM-A240. The former corresponds to S32304 (typical composition 23Cr-4Ni-0.17N), while the latter corresponds to S32101 (typical composition 22Cr-1.5Ni-5Mn-0.22N). The main types of steel in conventional duplex stainless steel were JIS SUS329J3L and SUS329J4L, but these are further higher in corrosion resistance than the SUS316L that is relatively high corrosion resistance type of an austenitic stainless steel and have expensive Ni and Mo added to about 6 to 7% (below, in the present invention, the % of the ingredients expressing mass %) and about 3 to 4% added to them respectively. As opposed to this, the lean duplex stainless steel is designed for a corrosion resistance of a level close to SUS316L or the general use steel SUS304, but instead makes the amount of addition of Mo substantially 0 and greatly reduces the addition of Ni to about 4% in S32304 and about 1% in S32101.
The duplex stainless steel described in Japanese Patent Publication (A) No. 2006-183129 is an improved version of the duplex stainless steel S32304 described in Japanese Patent Publication (A) No. 61-56267 in which the corrosion resistance in an acidic environment is raised by adding Cu and the strength is raised by adding any of Nb, V, and Ti. Further, Japanese Patent Publication (A) No. 2006-183129 prescribes a type of ingredients of a lean duplex steel as an austenitic/ferritic stainless steel superior in ductility and deep drawability in which, as a selective element, 0.5% or less of V is added and in which, as an effect, the micro-structure of the steel is refined and the strength is raised.